1. Field of the Invention
The present invention relates to a wide-angle lens system for use in a surveillance camera or an in-vehicle camera, etc., and in particular, relates to an inexpensive wide-angle lens system having a four-lens-element arrangement for in-vehicle use for detecting white lines on roads and obstacles by projecting near-infrared light in addition to visible light.
2. Description of the Prior Art
In recent years, due to developments in ITS (Intelligent Transport Systems), sensor cameras for driving support are used in addition to conventional back monitor cameras. Furthermore, in addition to normal headlights, a sensor-camera-installed vehicle is provided with a near-infrared projector which can be used in night driving; and an imaging device, such as a CCD or a CMOS, etc., which is arranged to have a practical sensitivity covering the near-infrared region, is used.
Accordingly, a lens system for such a sensor camera is required to have at least the following features:
(i) the lens system has a wider angle-of-view;
(ii) chromatic-correction is adequately made from the visible light region to the near-infrared light region;
(iii) the lens system is usable even in a temperature change from approximately −30° C. to approximately +90° C. since the sensor camera is used inside a vehicle; and
(iv) the lens system is inexpensive.
However, in a conventional wide-angle lens system which is originally designed to be used with a general infrared-cut filter, the correcting of chromatic aberration is insufficient. Consequently, such a conventional wide-angle lens system cannot be used in an in-vehicle sensor camera.
Furthermore, a surveillance camera lens system, e.g., as disclosed in Japanese Unexamined Patent Publication No. 2006-3589, which is designed to be used in the near-infrared region is known in the art; however, most of the surveillance camera lens systems are designed for variable-focal-length lens systems, have a large number of lens elements, and are not supplied in the market at a lower price.
In addition, Japanese Unexamined Patent Publication No. 2004-354829 discloses a single focal-length lens system which is designed to consider (the correcting of) chromatic aberration up to the near-infrared region; however, this lens system is constituted by a larger number of lens elements, i.e., seven lens elements, and is not supplied in the market at a lower price due to the use of cemented lens elements. Moreover, under a large temperature change, there would be a possibility that cemented lens elements are detached from each other.
In both of Japanese Unexamined Patent Publication Nos. H05-264895 and H10-39207, a four-lens-element arrangement is taught, and cemented lens element are not employed; however, the correcting of chromatic aberration cannot be made in the near-infrared region.